Method of positioning electrodes for central nervous system monitoring and sensing pain reactions of a patient

ABSTRACT

A method concerning measurements by using an electrode array comprising three electrodes for central nervous system (CNS) monitoring from the forehead of a patient&#39;s head. A first electrode of said three electrodes is positioned between the eyebrows or immediately above the eyebrows of the patient. A third electrode of said three electrodes is positioned apart from the first electrode on the hairless fronto-lateral area of the forehead the patient. A second electrode is positioned between the first and the third electrodes on the forehead of the patient.

The present invention relates to a method for central nervous system(CNS) monitoring, and more specifically to a method of positioningelectrodes in an electrode array comprising three electrodes formonitoring central nervous system (CNS), ie electroencephalography (EEG)and frontal electromyography (FEMG) signals from the forehead of apatient's head. The invention also relates to a method of sensing painreactions of a patient.

Electroencephalography (EEG) is a well-established method for assessingthe brain function by picking up the weak signals generated in the brainwith electrodes on the skull surface. To obtain the signals, multipleelectrodes are placed on the scalp of a patient in accordance with arecognized protocol. EEG has been in wide use for decades in basicresearch of the neural system of brain as well as clinically indiagnosis of various neurophysiological disorders.

In a traditional EEG measurement electrodes are attached following thestandard 10-20 system. Said system has been used by neurophysiologistsfor decades to record EEG and to find pathological EEG changes. Thesystem however requires cumbersome attachment of multiple electrodes,especially when the electrodes are attached in the hair environment.

One of the special applications for EEG which has received attention toduring the 1990's is use of a processed EEG signal for objectivequantification of the amount of brain activity for the purpose ofdetermining the level of consciousness of a patient. In its simplestform, the usage of EEG allows for the automatic detection of thealertness of an individual, ie. if he or she is awake or asleep. Thishas become a significant issue, both scientifically and commercially, inthe context of measuring the depth of unconsciousness induced byanesthesia during surgery. Modern anesthesia practices use asophisticated balanced anesthesia technique with a combination of drugsfor maintaining adequate hypnosis, analgesia, muscle relaxation, andsuppression of the autonomic nervous system. The need for a reliablesystem for monitoring of the adequacy of the anesthesia is based on bothsafety and economical concerns. An anesthesia dose which is too lightcan, in the worst case, cause the patient wake up in the middle of theoperation and create a highly traumatic experience both for the patientand for the personnel administering the anesthesia. At the oppositeextreme, the administration of too much anesthesia generates increasedcosts due to the excessive use of anesthesia drugs and the time neededto administer the drugs. Over dosage of the anesthesia drugs alsoaffects the quality and length of the post-operative period immediatelyafter the operation and the time required for any long-termpost-operative care.

In the anesthesia and the intensive care environment said 10-20 systemis very rarely used. This is because these environments are alreadycrowded by many other measuring systems, such as blood pressure, ECG,inspired and expired gas measurements. The additional labour-consumingmeasuring system would take too much time and effort from the carepersonnel. There is even though need for central nervous systemmonitoring in these areas. The consciousness level of the patient isvaried in both of said environments and till today there has not been apractical method for monitoring the level of consciousness in theanesthesia and the intensive care environment.

As told before in the anesthesia environment patient is anesthetizedwith hypnotic, analgesic and neuromuscular blocking agents. Theneuromuscular blocking agents, given in a certain extent block theneuromuscular junction and the patient looses ability to moveherself/himself. This can create a situation where patient feels painbut cannot communicate. Without central nervous system monitoring thereis a risk of giving too little or too much anesthetics. If too littlehypnotic drugs is given to the patient she/he could awake duringoperation, which could cause traumatic experience especially for thepatient and also for the personnel. On the other hand over dosage ofhypnotic drugs affects the quality and length of the post-operativeperiod.

The above mentioned reasons have generated commercial efforts to developEEG devices to said environments during the past ten years. The mainrequirements for such monitoring can be described by the followingfeatures, ease of use, reliability and good quality. The efforts in thisarea have concentrated into reliable and easy electrodes as well as togood quality signal processing.

A significant advancement in making the EEG-based measurement of theadequacy of anesthesia an easy-to-use, routine was a finding based onPositron Emission Tomography (PET) that determined that the effects ofthe anesthetic drugs on the brain are global in nature. This means thatfor many applications it is enough to measure the forebrain or frontalcortex EEG from the forehead of the patient. The forehead is both aneasy to access and is hairless location on the patient. Electrodesplaced with an appropriate spacing between the electrodes on theforehead can pick up an adequate signal originating from the anteriorcortex in the brain.

Since the Positron Emission Tomography (PET) studies have shown that theanesthesia effect is a global phenomena in the brain, the sensordevelopment efforts have concentrated on the hairless frontal area ofthe head. The first commercial sensor for this application area wasdeveloped by the company Aspect Medical Systems, Inc. U.S. Pat. No.6,032,064 can be mentioned as an example of the art describing thesensor developed by Aspect Medical Systems, Inc. The company mentionedabove also has patented many electrode configurations relating toplacement of the electrodes on frontal and temple areas of the patient'shead. Reference is made here to U.S. Pat. No. 6,394,953.

While the foregoing has discussed the use of EEG signals, it is alsodesirable to obtain frontal electromyographic (FEMG) signals arisingfrom the forehead of the patient. The frontalis muscle is the firstindicator of approaching consciousness. When this muscle activity issensed by appropriately placed electrodes it provides an earlyindication that the patient is emerging from anesthesia. Similarly theseelectrodes can sense pain reactions originating from this same muscleactivity when the anesthesia is not adequate, for example because ofinadequate analgesia. So the FEMG signals give an early warning ofarousal and may also indicate inadequate analgesia.

The object of the invention is to provide a simple and practical methodof positioning an electrode array so that the electrodes of the arrayare optimally located for recording EEG and FEMG signals.

An advantage of the invention is in that the method is extremely simple,practical and reliable, and therefore optimal measuring results can beobtained. Another advantage of the invention is in that the method canbe materialized very simply, ie. by using a simple electrode array,whereby costs can be kept at reasonably low level.

In the following the invention will be described in greater detail bymeans of the examples described in the attached drawing, in which

FIG. 1 shows a frontal view of a human head where electrodes arepositioned according to the first embodiment of the invention,

FIG. 2 shows a frontal view of a human head where elecrodes arepositioned according to the second embodiment of the invention,

FIG. 3 is a diagram showing State Entropy (SE) and Response entropy (RE)when the patient is falling asleep,

FIG. 4 shows a frontal view of a human head with alternative positionsof additional electrodes and

FIG. 5 is a plan view of the electrode array of the system shown inFIGS. 1, 2 and 4.

Referring now to the figures in which corresponding details in differentembodiments have been marked with same reference numerals, the sensormeasurement system of the present invention is indicated generally at 1in FIG. 1. The system 1 includes an electrode array 2 connected to amonitor 3 by a cable 4. The array 2 transmits central nervous system(CNS) signals from the forehead 5 of the patient to the monitor 3, whichcarries out signal processing and displays EEG and FEMG data in desiredform. The data obtained can also be stored for future use.

The electrode array 2 shown in FIG. 1 comprises three electrodes, ie.the first electrode 6, the second electrode 7 and the third electrode 8.The structure of the array 2 is shown more clearly in FIG. 5. Theelectrodes 6, 7 and 8 have been connected to a connector 9 by usingconductors 10, 11 and 12. The electrodes and the conductors have beenplaced on a flexible substrate 13 made of an appropriate material, forexample plastic material. The shape of the substrate can comprise threemain bodies connected to each other by narrow extensions as shown inFIG. 5. Said form is advantageous because narrow extensions enable thatpositions of the electrodes when compared to each other can be variedaccording to the existing need when the array is placed on the foreheadof the patient. It is however quite possible that the substrate isformed to be essentially of rectangular shape etc.

The first electrode 6 is used to measure phasic and tonic activation offacial muscles intended for expression of painful mimic responses(corrugator, procerus, frontalis and orbicularis oculi muscles), theFEMG signal. The first electrode 6 measures also some EEG relatedsignal. The third electrode 8 measures cortical activity (EEG) of eitherfrontal lobe from the hairless fronto-lateral area and only some FEMGrelated signal. The second electrode 7 is a ground electrode.

According to the basic idea of the invention the first electrode 6 ispositioned between the eyebrows of the patient as shown in FIG. 1 orimmediately above either of the eyebrows, ie. above the eyebrows nearfrontalis and orbicularis muscles of the patient, as shown in FIG. 2.The third electrode 8 is positioned apart from the first electrode 6 onthe hairless fronto-lateral area of the frontal lobe of the patient asshown in FIGS. 1 and 2. Preferably the third electrode 8 is positionedas far as possible from the first electrode 6. The second electrode 7 ispositioned between the first electrode 6 and the third electrode 8 onthe forehead of the patient as shown in FIGS. 1 and 2. It is veryadvantageous to place the second electrode 7 essentially in the middleof the first and the second electrodes so that the distances between thesecond electrode 7 and the first electrode 6, and the second electrode 7and the third electrode 8 are essentially the same. This gives theopportunity to optimize Common Mode Rejection Ratio (CMRR) of themeasured signal.

The three-electrode system described above and placed according to thebasic idea of the present invention offers a simple solution to optimizeboth the EEG and FEMG signals at the same time. This is due to the factthat EEG and FEMG signals are best recorded when both measuringelectrodes, ie. the first electrode 6 and the third electrode 8 aresituated on the forehead of the patient. The first electrode 6 is placedon the area of the forehead in which the FEMG signal is stronger thanthe EEG signal. The third electrode 8 is placed on the area of theforehead in which the EEG signal is stronger than the FEMG signal. EEGsignals of the hairless area of the forehead arise from thefronto-lateral area of the frontal lobe of the patient. FEMG signalsarise from the forehead between the eyebrows or just above the eyebrowsnear frontalis and orbicularis muscles of the patient, ie. on the areawhere the facial muscles intended for expression of painful responsesare situated. In other words one electrode collects a particularly highEMG-contribution from the muscle-rich region close to the eyebrows,while the voltage difference between the two electrodes over thefronto-lateral area measures a high EEG signal component. When measuringpain reactions the electrodes can be placed more freely than describedabove. For example the first electrode 6 would be placed as told aboveto enhance the FEMG signal. The other electrodes, the second one 7 andthe third one 8 can be placed on the hairless areas of the head, forexample frontal lobe, temples, cheeks, ears or areas around the ears.The third electrode 8 can be positioned apart from the first electrode 6and the second electrode 7 can be positioned between the first and thesecond electrodes. Preferably however the second electrode 7 is placedin the middle of the first electrode 6 and the third electrode 8. Inthis connection it is important to realize that the embodiments shown inthe Figures are suitable also for measuring pain reactions. As toldabove the FEMG signals give an early warning or indication of painreactions of the patient, ie. according to the basic idea of theinvention said signals can advantageously be used for better detectingemerging arousal and possible inadequate analgesia. When measuring painreactions the EEG signal is not necessarily needed. When the EEG signalis not used the third electrode can act for example as a referenceelectrode. It is however advantageous to monitor also the EEG signalwhen pain reactions are measured.

The EEG and FEMG signals can be recorded with only three electrodessituated on the forehead of the patient. This simple three-electrodesystem situated according to the basic idea of the invention on theforehead offers a practical possibility to optimise both the EEG andFEMG signals at the same time.

The EEG and FEMG signals obtained from the patient in the way asdescribed above can very advantageously be processed to monitor thestate of consciousness by using State Entropy parameters, ie. StateEntropy (SE) and Response Entropy (RE). State Entropy corresponds toentropy of the EEG signal and Response Entropy corresponds to entropy ofthe EEG and FEMG. The idea of RE and SE indices can be seen in FIG. 3.The FEMG signal, ie. RE in FIG. 3, is dominating when the patient isawake and ceases when the patient falls asleep. Then EEG signal, ie. SEin FIG. 3, becomes dominant and the indices RE and SE have the samevalues. During an operation there can also emerge certain periods whenthe RE rises. These situations can be due to inadequate analgesia, noiseor artefacts in the signal. So both of the measuring sites are needed tooptimize RE and SE information. The electrode configuration shown inFIG. 2 can also be used to record CNS activity of both frontal lobes atthe same time.

It is also possible to use one or more additional electrodes togetherwith the electrode array described above. Said additional electrodes canbe positioned on the area surrounding an eye of the patient in order torecord the movements of the eye and to eliminate disturbances caused bysaid movements. The additional electrodes can be positioned above or/andunder the eye or/and adjacent to the eye as shown by reference numeral13 in FIG. 4. In the embodiment of FIG. 4 two additional electrodes 14,15 are used. The additional electrodes are connected by using cables 16,17 to the monitor 3. It is also possible to connect the additionalelectrodes to the monitor via connector 9 by designing the connector tohave some additional inputs. In this connection it must be noted thatFIG. 4 shows only one eventual embodiment, it is quite possible also toposition the additional electrodes above and under the eye or under andadjacent to the eye etc. It is further possible to use only oneadditional electrode positioned on one of the positions 13, but betterresults can be achieved by using two additional electrodes.

The embodiments described above are by no means intended to limit theinvention, but the invention may be modified completely freely withinthe scope of the claims. Thus it is obvious that the details need not beexactly identical with those shown in the Figures and described in thetext, but other solutions are also possible. For example the positionsof the electrodes shown in Figures are not the only possible positions,but it is quite possible within the spirit of the invention to use alsoslightly different positions etc.

1-9. (canceled)
 10. Method of sensing pain reactions of a patient inwhich method an array comprising three electrodes for central nervoussystem (CNS) monitoring comprising frontal electromyography (FEMG)signal from a patient's head, said method comprising the steps of:positioning a first electrode of said three electrodes on the hairlessarea of the patient's head in which the FEMG signal is strong;positioning a third electrode of said three electrodes on the hairlessarea of the patient's head; positioning a second electrode on thehairless area of the patient's head; and recording mainly frontalelectromyography (FEMG) signals by using the first electrode to obtainan early indication of pain reactions.
 11. The method of claim 10, inwhich method the central nervous system (CNS) monitoring compriseselectroencephalography (EEG) signal from the patient's head and in whichmethod the first electrode is positioned on the area of the patient'shead in which the FEMG signal is stronger than the EEG signal and thethird electrode is positioned on the area of the patient's head in whichthe EEG signal is stronger than the FEMG signal.
 12. The method of claim10, in which the first electrode is positioned between the eyebrows orabove the eyebrows near frontalis and orbicularis muscles of thepatient, the third electrode is positioned apart from the firstelectrode on the hairless fronto-lateral area of the frontal lobe of thepatient, and the second electrode is positioned between the first andthe third electrodes.
 13. The method of claim 10, in which the first andthe third electrodes are measuring electrodes and the second electrodeis a ground electrode.
 14. The method of claim 13, in which the secondelectrode is positioned essentially in the middle of the first and thesecond electrodes having essentially the same distance to said first andthird electrodes.
 15. The method of claim 10, the method furthercomprising the step of positioning at least one additional electrode onthe area surrounding the eye of the patient to monitor the movements ofthe eye.
 16. The method of claim 15, in which two additional electrodesare used.
 17. The method of claim 15, in which the additional electrodesare positioned above or/and under the eye or/and adjacent to the eye.18. The method of claim 11, in which the first electrode is positionedbetween the eyebrows or above the eyebrows near frontalis andorbicularis muscles of the patient, the third electrode is positionedapart from the first electrode on the hairless fronto-lateral area ofthe frontal lobe of the patient, and the second electrode is positionedbetween the first and the third electrodes.
 19. The method of claim 11,in which the first and the third electrodes are measuring electrodes andthe second electrode is a ground electrode.
 10. The method of claim 12,in which the first and the third electrodes are measuring electrodes andthe second electrode is a ground electrode.
 21. The method of claim 16,in which the additional electrodes are positioned above or/and under theeye or/and adjacent to the eye.